Hearing aids to be carried in the auditory canal, so called CIC-appliances (CIC for "Completely In the Canal"), comprise like other hearing aids substantially the following components serving the hearing function: a microphone directed toward the outside of the auditory canal for reception of sound from the outside world and for transformation of this sound into electric signals, an amplifier for amplification of these electric signals, a loudspeaker facing the inside of the auditory canal for transformation of the amplified electric signals into sound and a power source (battery) for operation of the named components. The amplifier is normally designed as an integrated circuit (chip) and, in addition to the amplifying functions, it may have control functions and/or may be programmable. The hearing aid may further comprise a receiving coil for reception of signals for remote control of the appliance or for reception of radio or telephone signals. The hearing aid can also be designed for the reception of radio signals only and thus only comprise a receiving coil and no microphone.
Furthermore, a CIC-hearing-aid needs supporting elements like any other hearing aid and is advantageously adapted in its form to the auditory canal of the individual person carrying it as precisely as possible.
ITC- and CIC-hearing-aids which are adapted to an individual auditory canal, by casting a body using the auditory canal or a model of it as a mold, are e.g. described in the publication EP-629101 and in a parallel application to the present application (U.S. application Ser. No. 08/899,415). In their general condition, i.e. in the condition not yet adapted to an individual auditory canal the appliances comprise a casting cavity substantially confined by an extensible membrane which cavity, for individualizing the appliance, is filled with a casting material through a pouring opening in the region of the outer face. The hearing aid has the form of an irregular frustum or cylinder with an outer face and an inner face and a circumferential surface, wherein the outer face is formed by a face plate, the inner face substantially by the outlet side of the loudspeaker and the circumferential surface by the body being covered by the membrane and adapted to the auditory canal.
The main difference of a CIC-hearing-aid compared to other hearing aids is the fact that it reaches much deeper into the auditory canal when worn. This position, deep in the auditory canal, has two main advantages one of which is an acoustic one, the other one an aesthetic one. The acoustic advantage is the fact that the space between the eardrum and the inner end of the hearing aid is smaller which improves the quality of sound and reduces the necessary amplification, i.e. the energy necessary for operation. The aesthetic advantage is the fact that the appliance is less visible. A difficulty which arises from the position deep in the auditory canal is constituted by the fact that the appliance reaches as far as the bony part of the auditory canal which bony part is extremely sensitive to contact.
Therefore, it is desirable for the comfort of the wearer for the CIC-hearing-aid to be adapted to the individual auditory canal as precisely as possible and at the same time for its innermost part to have as little contact as possible with the wall of the auditory canal.